1. Field of the Invention
The present invention relates to a control apparatus for a device having a dead band. In the dead band, the device is not responsive fully or substantially to an input. That is, the device has non-linear characteristics. The present invention also relates to a variable valve timing system that has a malfunction-determining means.
2. Description of Related Art
A control system has a configuration for controlling a control object to a target state. In the case of a control object having a dead band, however, the response of the control object is non-linear so that it is difficult to realize both a fast response and control stability at the same time. It is particularly hard to control such a control object in areas in close proximity to boundaries of the dead band.
As an example of such a control object, a variable valve timing apparatus is known. JP-A No. H7-19073, JP-A No. H8-74530 and JP-A No. H8-109840 each disclose a variable valve timing apparatus for changing a valve timing of an engine. As an apparatus similar to such a variable valve timing apparatus, there is known a variable valve timing apparatus for changing a variable quantity of an intake valve and/or a variable quantity of an exhaust valve. Examples of the variable quantity are at least one of a valve timing, a lift magnitude, a working angle, and a combination of them. These apparatus are useful for, among others, increasing an output, reducing the amount of consumed fuel and decreasing exhaust emission.
Each of these apparatus typically employs an oil-pressure control valve such as a 3-port 3-position valve. As shown in FIG. 26A, a dead band exists in the middle of a characteristic. In this dead band, the response of an oil flow rate is very slow or there is all but no response even if a duty value Duty is changed. For this reason, a dead band DB appears also in the changing speed of a valve timing as shown in FIG. 26B. This dead band DB appears in an area in close proximity to a holding duty value Dh for holding a valve timing. FIGS. 26A and 26B are each a diagram showing a plurality of responses each obtained for an oil temperature. A solid line represents a response for an oil temperature of 80 degrees Celsius. A single-dotted line represents a response for an oil temperature of 20 degrees Celsius and a double-dotted line represents a response for an oil temperature of −10 degrees Celsius. The plus direction of the vertical axis of FIG. 26B represents advancing while the minus direction represents retarding. As shown in FIG. 26B, the width of the dead band DB changes in accordance with the temperature of oil.
As is obvious from the characteristics shown in FIGS. 26A and 26B, when the duty value Duty is changed from the holding duty value Dh in the advance or retard direction, the response of the valve timing is slow due to the dead band. In order to compensate the characteristic for the slow response, the duty value is corrected by using a feedback loop. With such correction, however, it is feared that an excessive response is resulted in right after a departure from the dead band.
In addition, since the apparatus utilizes engine oil, a delay time increases due to a decrease in oil pressure. Typically, the oil pressure decreases at a low temperature or when the revolution speed of the engine is low. Variations in delay time serve as a hindrance to the function of a means for determining whether the apparatus is normal or abnormal. JP-A No. H7-127407 discloses an apparatus, which operates to inhibit the determination of an abnormality when the discharge pressure of an oil pump drops to a level equal to or lower than a predetermined value.
After a long stopped state of the engine or after oil replacement, however, oil is inevitably lost from an oil-pressure circuit in some cases. In such a case, it takes more time to fill up the oil-pressure circuit with oil even if the discharge pressure of the oil pump has reached the predetermined value. Thus, during that time, the variable valve apparatus is not capable of responding normally. In consequence, with the disclosed technologies, it is quite within the bounds of possibility that a response delay caused by the loss of oil is determined incorrectly to be an abnormality of the variable-valve apparatus in spite of the fact that the apparatus itself operates normally.